1. Field of the Invention
The present invention relates to a power supply device, and particularly to a power supply device for performing DCxe2x80x94DC power conversion by use of capacitors.
This application is a counterpart of Japanese Patent Application, Serial Number 125517/2001, filed Apr. 24, 2001, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
A switched capacitor (hereinafter abbreviated as xe2x80x9cSCxe2x80x9d) type power supply device comprising a plurality of capacitors and a plurality of switches for respectively changing connected states of these capacitors is easy to bring into less size because it needs no transformers and coils. According to this type of SC type power supply device, a system on-chip including the power supply device can be implemented and hence a reduction in the cost of a system can be expected. Technologies related to conventional SC type power supply devices have been described in the following references.
Reference 1: National Semiconductor Corporation, xe2x80x9cLM3352 Regulated 200 mA Buck-Boost Switched Capacitor DC/DC Converterxe2x80x9d, Product Cataloguexe2x80x9d, March 1999
Reference 2: David H. Soo, National Semiconductor Corporation, xe2x80x9cSYSTEM AND METHOD FOR DUAL MODE DCxe2x80x94DC POWER CONVERSIONxe2x80x9d, U.S. Pat. No. 5,548,206.
When the SC type power supply device is used as one for step-down (input voltage greater than output voltage), n capacitors included in the SC type power supply device are electrically connected in series between a power supply (VDD) and ground (GND) in a charging cycle. At this time, the respective capacitors are charged by a voltage VDD/n. In a subsequent discharging cycle, the n capacitors are electrically connected in parallel between a voltage output terminal of the SC type power supply device and ground. Thus, the voltage VDD/n is outputted from the voltage output terminal. By repeating the charging cycle and the discharging cycle at high speed, the SC type power supply device is capable of outputting the voltage VDD/n from the voltage output terminal on a steady basis.
Thus, according to the conventional SC type power supply device, the value of an output voltage Vout can also be fixed on a circuit basis according to the number of capacitors (n) and the contents of their connections. As an alternative, however, the value thereof may be adjusted so as to coincide with a reference voltage Vref (xe2x89xa6VDD/n). In this case, however, the output voltage Vout and the reference voltage Vref are compared with each other by means of an operational amplifier or the like. When the output voltage Vout becomes lower than the reference voltage Vref, it is necessary to perform a so-called intermittent switching operation for making the discharging cycle effective.
The contents of circuits for the conventional SC type power supply device is shown in FIG. 8, and operating waveforms thereof are illustrated in FIG. 9.
An operational amplifier 600 compares an output voltage VOUT with a reference voltage VREF brought to a desired output voltage. DCHG switches 603 and 604 are respectively brought to a conducting state when VOUTxe2x89xa6VREF and only during the discharging cycle. Since the output voltage VOUT obtained as a result thereof is compared with the reference voltage VREF by means of the operational amplifier 600, the output voltage is adjusted so as to take the output voltage VOUT=reference voltage VREF.
During the charging cycle, a voltage dividing capacitor 605 and an output capacitor 606 are electrically connected in series between the power supply voltage VDD and ground GND. At this time, the voltage dividing capacitor 605 is charged by a potential of (power supply voltage VDDxe2x88x92output voltage VOUT), whereas the output capacitor 606 is charged by the output voltage VOUT. On the other hand, the voltage dividing capacitor 605 and the output capacitor 606 are electrically connected in parallel between the output voltage VOUT and ground GND during the discharging cycle. At this time, the voltage dividing capacitor 605 is discharged while the difference (power supply voltage VDDxe2x88x92output voltage VOUT) in potential between its both terminals is being held.
According to the conventional SC type power supply device as described above, when the reference voltage VREF for obtaining the desired output voltage value is set lower than the stable output voltage obtained by the series/parallel connection switching operations of the voltage dividing capacitor 605 and the output capacitor 606, the discharging operations of both capacitors are intermittently performed. In this case, the voltages applied across the respective capacitors during the charging and discharging cycles cannot be kept in equilibrium, thus resulting in the following problems.
(1) Since capacitors different in voltage are connected in parallel during a discharging cycle, a spike current flows between the capacitors, so that switching noise is produced.
(2) A ripple voltage in output voltage increases.
(3) Since a terminal voltage on the negative side of each capacitor is depressed low as compared with ground GND during the discharging cycle, the potential on a ground GND line varies, thus exerting a bad influence on other circuit characteristics.
(4) Since the terminal voltage on the negative side of each capacitor is depressed as compared with ground ND during the discharging cycle, a parasitic PN diode of an NMOS transistor is taken in he forward direction, thereby varying a characteristic as a power supply circuit.
The present invention has been made in view of the above problems. It is an object of the present invention to provide a power supply device which stably outputs a voltage adjusted to a desired value and takes into full consideration even electrical influences over other circuits.
In order to solve the above problems, the present invention provides a power supply device equipped with a switched capacitor type power supply circuit and an internal node voltage adjusting circuit. The switched capacitor type power supply circuit charges a plurality of built-in capacitors by use of an internal node voltage during a charging cycle and discharges the plurality of charged capacitors during a discharging cycle to thereby generate an output voltage corresponding to the internal node voltage. Further, the internal node voltage adjusting circuit compares the output voltage and a reference voltage adjusted to a predetermined level and adjusts the internal node voltage according to the result of comparison. The present power supply device changes connected states of a plurality of the capacitors to a series connection or a parallel connection during the charging cycle and the discharging cycle to thereby perform DCxe2x80x94DC power conversion. A switching pulse use for performing series/parallel connection switching operations of the respective capacitors is kept constant in duty ratio (i.e., time intervals for the charging cycle and the discharging cycle are substantially made equal to each other), so that the differences in potential between both terminals of the respective capacitors in the switched capacitor type power supply circuit are kept substantially identical to one another even during both the charging cycle and the discharging cycle.
Preferably, the switched capacitor type power supply circuit detects the value of an output current generated based on the output voltage and stops the operation of charging the plurality of capacitors and the operation of discharging them when the detected value is smaller than a predetermined value. According to such a configuration, power consumed by the power supply device can be reduced.
There is further provided output voltage selecting means for selecting any of the internal node voltage, the output voltage and a second output voltage and outputting it to the outside. Consequently, an improvement in power conversion efficiency is realized.
Further, it is preferable to provide a second switched capacitor type power supply circuit for charging a plurality of built-in second capacitors by using an internal node voltage during a charging cycle and discharging the charged second capacitors during a discharging cycle to thereby generate a second output voltage corresponding to the internal node voltage, and power supply circuit selecting means for selecting either the switched capacitor type power supply circuit or the second switched capacitor type power supply circuit and bringing it to an enable state. The power supply circuit selecting means is configured so as to select either the switched capacitor type power supply circuit or the second switched capacitor type power supply circuit with the level of the reference voltage as the reference. According to such a configuration, a further improvement in power conversion efficiency is achieved.
The second switched capacitor type power supply circuit is configured so as to detect the value of a second output current generated based on the second output voltage and stop charging and discharging operations for the plurality of second capacitors when the detected value is smaller than a predetermined value (see claim 11). According to such a configuration, power consumption of the power supply device can be reduced.
The internal node voltage adjusting circuit (resistor step-down type power supply circuit) may preferably be provided with internal node voltage output means for outputting an internal node voltage during period other than the discharging cycle. A transistor may be adopted as the internal node voltage output means. The transistor directly assumes the role as a discharging switch for the plurality of capacitors included in the switched capacitor type power supply circuit in addition to the execution of the role of outputting the internal node voltage. Accordingly, the power supply device is simplified in circuit configuration and made compact.